I have taken school groups to WDW for the past 14 years and have always used Mears when we need transportation to Kennedy Space Center or SeaWorld (no longer needed to the airport and back). The first year, we booked everything through a travel company and stayed offsite. . . never again***. The safety, convenience, and theming of the WDW resorts is enough to keep us on property. Also, WDW books our rooms in a block (adjacent to each other), and that first year we were scattered all over in the off-site hotel. The food courts at the Disney All-Stars are also very convenient for groups when dining. But back to your original question, Mears has always been super reliable, punctual, and the drivers are helpful and courteous. That's why we have stayed with them all these years. Staying onsite, the rooms don't cost that much more (pm me if you need a WDW Youth Group Planner phone number) and you also have the benefit of Extra Magic Hours.
***Among things mentioned, here's what ultimately made us realize that onsite is the way to go . . . our first year, we were in Epcot on a spring evening when the sky turned BLACK . . . we knew a storm was coming but had no way to get back to our offsite hotel, as our bus wasn't scheduled to pick us up until park closing!!! We were stuck there, and all of us got completely SOAKED! Two years ago, same thing happened and we simply exited the park, hopped onto a Disney bus, and got back to our resort before the storm hit.
